Brown crushed garlic in olive oil or margarine; add a spoon of flour and toast to make a roux. Then pour in a cup of non-dairy milk (I prefer cashew-based Mimicreme but have had success with soy). Season with salt (to taste depending on whether you used margarine).

It's possible to make a vegan "cheese" sauce out of potatoes, onions, garlic, and carrots cooked together, then pureed with soymilk (or water) vegetable margarine, cashews, lemon juice, mustard, salt and pepper. See here for a vegan mac and cheese recipe featuring this sauce:
http://www.vegnews.com/web/articles/page.do?catId=10&pageId=40
You can also make a vegan "alfredo" with nutritional yeast.